A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating, and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake, and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all of which are intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
In order to be able to image smaller features, it has been proposed to use extreme ultraviolet radiation (EUV) with a wavelength in the range of 5 to 20 nm, particularly 13 nm, or a charged particle beam, e.g. an ion beam or an electron beam, as the exposure radiation in a lithographic apparatus. These types of radiation require that the beam path in the apparatus be evacuated to avoid beam scatter and absorption. Because there is no known material suitable for making a refractive optical element for EUV radiation, EUV lithographic apparatus must use mirrors in the radiation (illumination) and projection system. Even multilayer mirrors for EUV radiation have relatively low reflectivities and are highly susceptible to contamination, further reducing their reflectivities and, hence, throughput of the apparatus. This imposes further requirements on the vacuum level that must be maintained and necessitates especially the hydrocarbon partial pressure to be kept very low.
At the same time, plasma radiation sources and the resist are substantial sources of contaminants that must be kept out of the illumination and projection systems. A discharge plasma source, for example, uses a discharge to create very hot partially ionized plasma, which emits EUV radiation. The plasma gas, which is often xenon (Xe), and debris from the source must be kept from entering the illumination system.
WO99/42904, incorporated herein by reference, discloses a contaminant trap, also called a filter, for trapping source debris. The known contaminant trap includes a plurality of foils or plates, which may be flat or conical and are arranged in a radial direction from the radiation source. The source, the filter, and the projection system may be arranged in a buffer gas, for example krypton, whose pressure is 0.5 torr. The contaminant particles then take on the temperature of the buffer gas, for example, room temperature, thereby sufficiently reducing the particles' velocities before they enter the filter. The pressure in the known contaminant trap is equal to that of its environment. This trap is arranged at 2 cm from the source, and its plates have a length, in the propagation direction of the radiation, of at least 1 cm and preferably 7 cm. This design requires relative large and thus costly collecting and guiding/shaping optics to bundle and shape the source radiation and guide it to the mask.
European Patent Application No. 01203899.8 describes a further improved device for trapping debris, such as may be emitted by a plasma source or from a resist exposed to EUV radiation. This document describes a contaminant trap including a first set of plate members arranged parallel to the direction of propagation of the radiation beam, and a second set of plate members that is arranged parallel to the direction of propagation. The first and second sets are spaced apart from one another along an optical axis of the radiation beam. There is a space between the first and second set of plate members. Flushing gas is supplied to that space to provide a high gas pressure to trap the contaminant particles. The two sets of plate members are designed such that leakage of the gas is minimized and that the gas pressure outside the trap is kept low. However, still, an amount of EUV is also absorbed by this gas with relatively high pressure.